The objectives are to enzymatically characterize peroxisomes from liver, kidney and other tissues of the body and to examine changes in enzyme activities of this subcellular organelle during post natal development and aging and after treatment with clofibrate, a hypolipidemic drug. Contribution to the methods of isolating peroxisomes and running assays on a Gilford autoenzyme analyzer have been made. To date catalase, uricase, alpha-hydroxy acid oxidase, a glyoxylate aminotransferase, carnitine acetyltransferase, carnitine octanoyltransferase and NAD:glycerol phosphate dehydrogenase have been studied in isolated liver peroxisomes and to some extent in kidney peroxisomes of the rat. Clofibrate increases total peroxisomal number and enzyme activity, and it increases the specific activity of the carnitine acyl transferase in the peroxisomes without affecting catalase and uricase. Work is in progress on the characterization of the isoenzyme form of glycerol phosphate dehydrogenase and its flavopeptide factor. The peroxisomal dehydrogenase but not the cytosol form of the enzyme is stimulated 10 fold by this flavopeptide. BIBLIOGRAPHIC REFERENCES: M.A.K. Markwell, L.L. Bieber, and N.E. Tolbert. 1977. The differential increase of hepatic, peroxisomal mitochondrial and microsomal carnitine acyltransferase in clofibrate-treated rats. Biochemical Pharmacology, in press. M.A.K. Markwell, S.M. Haas. 1977. Automated assay of enzyme markers for subcellular particles in study of clofibrate-treated rats. Fed. Proceed., in press.